PROJECT SUMMARY The dentate gyrus (DG) is a subfield of the hippocampus that has been implicated in multiple psychiatric diseases, including schizophrenia. Functions of the DG include pattern separation, novelty detection, and the expression of innate anxiety, which have all been disrupted in patients with schizophrenia and in schizophrenia mouse models. While neuromodulators including dopamine are thought to be involved in the pathogenesis of hippocampal dysfunction in schizophrenia, the biological mechanisms that underlie defects in DG-dependent behaviors in schizophrenia are unknown. Most studies of DG function and its role in disease have examined the DG principal cells, granule cells (GCs), and their unique ability to undergo adult neurogenesis. The DG also contains a second population of excitatory neurons called mossy cells (MCs), which have unique anatomical properties and extensive hippocampal connections that position them to modulate DG activity and affect behavior. They have also recently been linked to schizophrenia, in which the schizophrenia-associated protein dysbindin-1 localized to MCs. The involvement of MCs in schizophrenia may be tied to dopamine, as they are the only hippocampal neurons to express the dopamine D2 receptor subtype, and dopamine was recently shown to have a prolonged excitatory effect on them. This is significant in light of the fact that antipsychotics act at the D2 receptor and produce hippocampal changes, suggesting that these changes may be mediated by MCs. Despite recent evidence that MCs contribute to clinically relevant DG functions and are involved in psychiatric disorders, including schizophrenia, so far there has been a lack of studies directly examining their role in DG circuitry and in behavior. This proposal will utilize a recently developed Cre-transgenic mouse line in combination with Cre-dependent viral vectors to answer a number of outstanding questions about dopamine inputs to MCs, the role of MCs in the DG circuit, and the link between MCs, dopamine, and behaviors that are dysfunctional in disorders, particularly schizophrenia. The first aim will use a modified rabies virus in MC- selective Drd2-Cre mice to perform monosynaptic retrograde tracing to specifically identify monosynaptic inputs to MCs, especially dopaminergic and neuromodulatory projections. Aim 2 will employ specific optogenetic activation and silencing of MCs in hippocampal slices of Drd2-Cre mice to determine the effect of MC activity on GC output, and the influence of dopamine on the MC ? GC synapse. The third aim will utilize DREADD-mediated silencing of MCs in Drd2-Cre mice in vivo, and probe the resulting effects on DG-linked behaviors that are influenced by dopamine and are impaired in schizophrenia. Overall, this research will greatly enhance our understanding of MC anatomy and function at both a circuit and behavioral level, and will help elucidate the role of MCs, dopamine, and DG dysfunction in the pathogenesis of disorders like schizophrenia.